The patterning of radiation sensitive polymeric films with high energy radiation flux such as photons, electrons, or ion beams is the principle means of defining high resolution circuitry found in semiconductor devices. The radiation sensitive films, often referred to as photoresists regardless of the radiation source, generally consist of multicomponent formulations that are coated onto a desired substrate such as a silicon wafer. The photoresist film is then exposed to radiation. The radiation is most commonly ultraviolet light at wavelengths of 436, 365, 257, 248, 193 or 157 nanometers (nm), or a beam of electrons or ions, or ‘soft’ x-ray radiation, also referred to as extreme ultraviolet (EUV) or x-rays. The radiation is exposed patternwise to induce a chemical transformation that renders the solubility of the exposed regions of the film different from that of the unexposed areas. The film is then heated to enhance this chemical transformation. After heating, the film is treated with an appropriate developer, usually a dilute, basic aqueous solution, such as aqueous tetraethylammonium hydroxide (TMAH) to develop the photoresist image on the wafer.
Typical photoresists contain a polymeric component and are generally comprised of a polymeric matrix, a radiation sensitive component, a casting solvent, and other performance enhancing additives. The highest performing photoresists in terms of sensitivity to radiation and resolution capability are “chemically-amplified” photoresists, allowing high resolution, high contrast and high sensitivity that are not generally provided by other photoresists. Chemically amplified photoresists are based on a catalytic mechanism that allows a relatively large number of chemical events such as, for example, deprotection reactions in the case of positive tone photoresists or crosslinking reactions in the case of negative tone photoresists, to be brought about by the application of a relatively low dose of radiation that induces formation of the catalyst, often a strong acid.
Most of the current positive resist compositions comprise aqueous base soluble functional groups that are sufficiently protected with acid labile groups so that the resist initially will not dissolve in a developer. During exposure to radiation, the photoacid generator (PAG) present in the resist composition produces strong acid, which then catalyzes the removal of the acid labile groups on heating (PEB). This process produces aqueous base soluble material in the exposed area which then is developed with a basic aqueous developer to produce the images. Many of the current photoresists contain aqueous base soluble carboxylic acid functional groups that are protected with acid labile groups to produce aqueous base insoluble ester. The mechanism of the acid-catalyzed deprotection of the esters in the resist is as follows. Water is not needed for this reaction to occur.

The ease of this deprotection reaction depends on the stability of the carbocation. That is, if the carbocation is more stable, the activation energy for the reaction will be lower which will lead to deprotection at a lower temperature. It was long recognized that the ring-size of cycloalkyl tosylates has a strong effect on the stability of carbocations (H. C. Brown et. al., JACS, 78, 2735 (1956)). The rates of acetolysis of cycloalkyl tosylates were reported in this paper.
Although chemically-amplified resists have been developed for 248, 193 and 157 nm lithography, certain barriers to achieving higher resolution and smaller feature sizes remain due to physical, processing and material limitations. One such phenomenon that arises for imaging in the sub-50 nm regime, resulting in diminished image integrity in the pattern, is referred to as “image blur” (see, e.g., Hinsberg et al., Proc. SPIE, (2000), 3999, 148). Image blur is generally thought to result from two contributing thermally driven factors: gradient-driven acid diffusion and reaction propagation, the result being a distortion in the developable image compared to the projected aerial image transferred onto the film. The key metric controlling the image blur is the ratio R=(average rate of acid catalyzed deprotection)/(average rate of acid diffusion). The greater the value of the ratio R, the lower the image blur (Hinsberg et al., Proc. SPIE, (2004), 5376, 21). If the activation energy for the deprotection reaction is less than that of the diffusion process, then a reduction in post exposure bake (PEB) temperature will increase the value of R. Therefore, in order to minimize the image blur and achieve higher resolution, there is a need to develop new polymers with lower activation energy (low-Ea) protecting groups. Such new polymers will have the deprotection chemistry occurring at significantly lower temperatures and therefore the processing temperature can be lower and hence limit the blur from photoacid diffusion.